A typical scheme for sensing the temperature on a silicon die is illustrated by the FIG. 1.
It makes use of a p-n junction of a transistor T operating as a “diode”. A current ibias is provided and the fluctuations of the voltage Vsense can be provided to an analog-to-digital converter ADC, so as to provide a measurement of the sensed temperature.
However, such a simple approach has many major drawbacks, especially because it relies on several serial elements. More specifically, each of these elements may be a source of errors: current source CS inaccuracy, PN junction variations, ADC inaccuracy, etc. Each error accumulates at each stage so as, at the end, the resulting measurement is clearly jeopardized.
Consequently, other schemes have been designed like for instant those based on the PTAT principle.
Voltage generating circuits are well known in the art and are used to provide a voltage output with defined characteristics. Among applications of such circuits are circuits adapted to provide an output that is proportional to a sensed absolute temperature. These circuits are known under the acronym PTAT (Proportional To Absolute Temperature).
FIG. 2 illustrates a typical embodiment of the PTAT principle.
The circuits is made of two branches, A, B, wherein each branch comprises a transistor, respectively TA, TB and two current sources CSA, CSB respectively.
The two current sources provides currents with intensity iA, iB respectively.
The two transistors TA, TB are connected in a diode configuration wherein the base of each transistor is connected to its collector, thereby forming PN junctions that are used for measuring temperature, as explained before. The junctions can have equal areas or unequal areas, resulting in equal or different charges, QA, QB respectively.
Then, the differential voltage ΔVAB between the emitters of the two transistors is proportional to the absolute temperature of the silicon die on which the transistors are implemented.
The voltage is given by the following equation:
      Δ    ⁢                  ⁢          V      AB        =                    k        ·        T            q        ·          ln      ⁡              (                              i            B                                i            A                          )                            Wherein:        K is the Boltzmann constant,        Q is the charge of the electron, and        T is the operating temperature in Kelvin.        
This equation has for instance been described in “A Simple Three-Terminal IC Bandgap Reference” of Paul Brokaw, in IEEE Journal of Solid-State Circuits, vol. sc-9, no. 6, December 1974.
The equation makes it clear that the voltage ΔVAB is proportional to the temperature T. The voltage ΔVAB can then be provided to an analog-to-digital convertor (ADC, not depicted).
The major issue is that the temperature variation is normally small so that the voltage ΔVAB is also small. This implies an important constraint to the analog-to-digital convertor which should be very accurate to reflect precisely the sensed voltage ΔVAB, especially in the low-figure domain.
In practice, due to the small figure of the voltage ΔVAB and its digital conversion, the resulting temperature measurement is poor.